This invention generally relates to the field of communications and, more particularly, to a method and apparatus which detects interference in a twisted wire pair caused by sources external to the transmission system and which eliminates or reduces the effects of the interference.
Twisted pairs of copper telephone wire, commonly referred to as twisted pairs, have been in existence since the late nineteenth century when Alexander Graham Bell first invented them. In fact, most of the transmission lines in the U.S. telephone network, commonly referred to as the telephone loop plant, are twisted pairs. Although much of the loop plant has been or is currently being upgraded with optical fiber, it is expected that twisted pair technology will remain in place for many years due to the high cost of upgrading and due to the ability to achieve high transmission rates over twisted pairs.
Digital subscriber line (DSL) technology, first developed to support basic rate integrated services digital network (ISDN) transmission over twisted pair, has made it possible to achieve high transmission rates of video, audio and data over twisted pairs. Today, there are many variations of DSL technology in use, such as asymmetric digital subscriber line (ADSL), high-bit-rate DSL (HDSL), and rate-adaptive DSL (RADSL), which are collectively classified under the rubric xe2x80x9cxDSLxe2x80x9d. All of these technologies correspond to line coding schemes which can be used to transmit and receive voice, video, and data over existing copper twisted pairs.
Twisted pairs attenuate signals in proportion to the length of the line and the frequency. In order to compensate for this characteristic, telephone companies design lines in accordance with certain standards designed to maximize performance, such as requiring that wires which extend over certain distances be a certain gauge and requiring that loading coils be installed on lines in many cases. Although, attenuation generally is the dominant factor affecting performance of twisted pairs, cross talk also affects performance. Cross talk corresponds to interference in a twisted pair caused by an adjacent twisted pair. Cross talk increases with frequency and with the number of cross-talking pairs. ADSL was developed to reduce the effects of cross talk. In accordance with ADSL technology, data is transmitted upstream at one rate and downstream at a much higher rate than the upstream rate, with the upstream and downstream data being separated through the use of frequency division multiplexing.
Another factor which affects the performance of twisted pairs is near and far end echo resulting from impedance differences created by telephone loop hybrid circuits. Telephone loop hybrid circuits are necessary in order to enable full duplexing of the signals transmitted between the end user and the central office. Echo cancellation techniques allow the echo to be calculated and canceled out to improve signal quality. Echo cancellation technology has now been widely implemented and most modems manufactured today contain an echo cancellation component.
Another factor which affects the performance of twisted pairs is noise or interference created by sources which are external to the telephone network. This interference may emanate from local area sources, such as electrical wiring within the customer premises, or from broad area sources, such as power lines which run along side the telephone lines within the loop plant and from radio transmitting stations. It would be beneficial to provide a method and apparatus capable of eliminating or reducing local area and broad area interference in a twisted pair transmission system in order to maximize performance of twisted pairs.
The present invention provides a method and apparatus for eliminating or reducing local area and broad area interference in a twisted pair transmission system. The apparatus of the present invention comprises a detection device, such as an antenna, for example, for acquiring representative electromagnetic interference coupled into a twisted pair line, a sampling/scaling device which samples and scales the acquired signal, and a combiner device which combines a signal correction component with the signal received over the twisted pair by a differential receiver.
In accordance with the preferred embodiment of the present invention, the sampling/scaling device comprises an analog-to-digital converter which converts the detected analog signal into a digital signal and a digital signal processor which receives the digital signal from the ADC and processes the signal to generate a correction signal. The correction signal is scaled in amplitude and phase such that, when subtracted from the received signal, it will cancel the additive interference that was present in the received signal. The correction signal is then subtracted from the signal received by the differential receiver from the twisted pair. The sampling/scaling device may be fixed or adaptive. If the interference is stationary in time, a fixed sampling/scaling device is suitable for use with the present invention. If the interference is non-stationary, preferably the sampling/scaling device will be adaptive such that the amount of scaling performed can be altered in accordance with changes in the interference signal detected by the detector.
The detector may be any device capable of detecting the interference, such as, for example, a capacitor, an antenna, or an induction coil, or a combination of two or more of these. The type of detector used will depend on the type of interference being coupled into the twisted pair transmission system.